An image display apparatus, which displays an image that is produced by enlarging by a projection lens an image formed by an image displayed on an image display element having a plurality of picture elements that can control light in accordance with image information, is widely used by being referred to as a front projector or a rear projector. As an image display element of such a kind, a CRT, a liquid crystal panel, DMD (digital micron mirror device (Registered Trade Mark): manufactured by Texas Instruments Incorporated) have been developed and used. Additionally, an inorganic EL, an inorganic LED, an organic LED, etc. are studied.
Moreover, as an image display apparatus, which does not display an enlarged image but display an image with an original scale, there have been developed and used a plasma display and a fluorescent display tube in addition to the above-mentioned CRT, liquid crystal panel, inorganic EL, inorganic LED, organic LED. Additionally, a field emission display (FED), a plasma addressing display (PALC), etc., are studied. Although they are generally classified into two types, a self-emission type and a spatial light modulator type, each has a plurality of picture elements that can control light.
A demand common to the above-mentioned image forming apparatuses is to achieve a high resolution, that is, to increase a number of picture elements. A display apparatus for HDTV having about one thousand scanning lines for broadcast use has already been on a market. Additionally, a development of a display apparatus having about two thousand scanning lines has been announce for the purpose of use as a work station computer with an increased resolution by using a liquid panel technique. However, increasing the number of picture elements may deteriorates a yield rate of a liquid crystal panel and may decrease an open area ratio. Thereby, there are problems that a cost is increased, brightness and contrast are decreased, and a power consumption is increased.
In order to solve the above-mentioned problems, Japanese Patent No. 2939826 and Japanese Laid-Open Patent Application No. 6-197297 disclose image display apparatuses that perform an interlace display having twice number of picture elements using a single image display element. Japanese Laid-Open Patent Application No. 7-36504 discloses a display apparatus having more than four times a number of picture elements using a single image display element. These display apparatuses are of a so-called picture element shift type, which is a method to increase an apparent number of picture elements by deflecting an optical path of a light projected from an image display element at a high speed with time division. Japanese Laid-Open Patent Application discloses a technique to increase a resolution by increasing an entire image by an optical path deflecting means.
Specifically, the projection display apparatus disclosed in Japanese Patent No. 2939826 comprises at least one optical element capable of rotating a deflection direction of a transmitting light and at least one transparent element having a birefringent effect in the middle of an optical path from a display element to a screen, wherein means for shifting an image formed by a display element within a projection optical path. Additionally, the projection display apparatus further comprises means for discretely projecting projection areas of picture elements of a display element by effectively reducing an open area ratio of the display element so as to make an attempt to improve contrast.
The liquid crystal projector disclosed in Japanese Laid-Open Patent Application No. 6-197297 is configured and arranged to change an inclination of a glass plate of a variable prism by a control of a control circuit, wherein when the glass plate inclines upward, an optical axis is refracted downward by a predetermined angle by being passed through the variable prism and when the glass plate inclines downward, the optical axis is refracted upward by a predetermined angle by being passed through the variable prism. That is, by changing the wedge shape configuration of the prism, an incident angle to the projection optical system is deflected by a minute angle so as to achieve image shifting.
By performing a control of a control circuit repeatedly in synchronization with a vertical synchronization signal or the like, a resolution of an image projected onto a screen is improved. In the image display apparatus disclosed in Japanese Laid-Open Patent Application No. 2002-139792, an amount of shift of an intersection between a screen and an optical axis before and after deflection by a light deflecting means is set to be equal to a width of an image piece that is a projected image on a line passing each intersection so as to display a plurality of image pieces by deflecting lights by the light deflecting means at a predetermined time interval. That is, a number of picture elements of an image formed by an optical modulation element is increased by the light deflecting means.
FIG. 1 shows a basic structure of a projection optical system of a conventional image display apparatus that shifts picture elements at a high speed by using an optical path deflecting element so as to apparently increasing a number of picture elements. FIG. 2 shows a positional relationship between a light valve and a projection lens.
In FIG. 1, an optical path deflecting element 2 is located between a light valve 1 and a projection optical system having projection lenses 5 and 6. A light passed through the optical path deflecting element is projected onto a screen 4. The optical path deflecting element 2 has a function to deflect the optical path by an angle corresponding to a half pitch of a displayed picture element pitch. In this case, BF is a back focus of the projection lenses of the projection optical system 3, D is an interval between the liquid crystal panel (light valve 1) and the optical path deflecting element 2 (D<BF), and P is an image picture pitch of the light valve 1.
Since the optical path deflecting angle is very small as compared to the interval D, if the deflecting angle is required to be Δθ for the optical path deflecting element 2, the following relationship is established.tan Δθ≈sin Δθ≈P/2D For example, if P=14 μm and D=30 mm, Δθ=48.1 seconds, i.e., Δθ=0.01336 degrees. Contrary, if Δθ is determined, an amount of shift ΔS is calculated by P/2=D−sin Δθ.
It is appreciated that a value of P/2 is in proportion to a value of D. That is, it is appreciated that a positional relationship between the light valve 1 and the optical path deflecting element 2 gives influence to the amount of shift ΔS.
On the other hand, it is possible to maintain a performance by eliminating an adjusting process by making parts tolerances and assembly tolerances severe when assembling light valves, various prisms and projection lenses to a housing of the projection apparatus in an assemble process of the conventional image forming apparatus. However, distributing the tolerances to achieve a performance by merely gathering parts may increase a part cost and an assemble cost, which is not practical.
Actually, a performance for projecting an image onto a screen is maintained by a so-called focus adjustment in which accuracy of various kinds of parts and accuracy of positioning have certain tolerances and the light valve, which finally corresponds to an object face, is minutely moved in an optical axis direction. In such a present condition, a consideration was given to a case where the optical path deflecting element 2 is located between the projection lens 5 and the light valve 1.
For example, when a deflecting element having D=30 mm, P=14 μm and Δθ=48.1 seconds is set, al focus adjustment range of is given to a panel so that a position setting error of the deflecting element is about ±0.5 mm. ±0.5 mm corresponds to a value of 3.4% of 30 mm.
Therefore, if a position adjustment of the panel in an optical axis direction is performed in a state where the deflecting element is fixed, an error of 3.2% of a shift error of 7 μm, that is, an error of 0.22 μm is generated. Although this value is very small, there may be a problem when lines are displayed with subtly different widths as to whether odd dots are ON or even dots are ON in a line and space display.
In FIG. 2, in order to explain the above-mentioned problem, the amount of shift is enhanced for the sake of easy recognition. The picture element at a position of “O” on the light valve 1 travels with a light flux indicated by solid lines toward the optical path deflecting element 2, and is switched toward two directions indicated by solid lines and dashed lines at a high speed, and as if an image of a picture element at a position “O′” is projected by the object lens onto the screen. An amount of shift is equal to a distance A shown in FIG. 2.
However, if the light valve 1 is adjusted to a position indicated by dashed lines by focal position registration of the light valve 1, an amount of shift of a picture element on the light valve 1 observed from the object lens 5 is A+Δ, and it can be appreciated that the amount of shift of the picture element is fluctuated. Moreover, since a time period of shifting the picture element depends on a drive period of the optical path deflecting element 2, there always is a display leak within the time period, and the leaking light gives an influence to degrade resolution. Although the light is cut in the conventional apparatus so as to prevent such a degradation in resolution, a further improvement is required in the viewpoint of effective use of light, and there is a room for improvement.
It should be noted that FIG. 3 is an illustration showing a conventional structure in which an optical path deflecting element is located between a projection optical system and an spatial light modulation element.